We propose to utilize pulsed EPR methods to study the structure and function of the tetranuclear Mn cluster of Photosystem II (PS II) of plant photosynthesis and the Mn catalase enzyme. Specifically, the techniques of electron spin echo envelope modulation (ESEEM) and electron spin echo - electron nuclear double resonance (ESE-ENDOR) will be utilized to detect nuclear spin transitions of magnetic nuclei within and in close proximity to these multinuclear Mn clusters. High frequency ESE-ENDOR will be utilized to measure the 55Mn spin transitions of the biological Mn clusters and of synthetic model compounds of known structure. Oriented PS ll membranes and single crystals of PS ll will be studied for enhanced resolution. These experiments will provide new knowledge about the structure of this essential catalytic site. The protein ligation to the Mn clusters in PS H and catalase will be explored. The binding of substrate and inhibitor molecules will be characterized. For the PS ll Mn cluster, experiments will be per- formed to characterize the roles of Ca2+ and Cl-, which are essential cofactors in the oxygen evolution chemistry. Experiments will be performed to test the hypothesis that photosynthetic oxygen evolution occurs with a metalloradical mechanism, involving both the Mn cluster and tyrosine Yz. Parallel polarization EPR signals will be used to detect integer spin signals of the Mn clusters.